The present invention relates to a process for reducing the level of corrosive impurities in sulfolane which has been used to extract aromatic hydrocarbons from petroleum. Sulfolane (tetrahydrothiophene 1,1-dioxide; also referred to as tetramethylene sulfone), is a highly polar compound having excellent solvent properties, very good chemical and thermal stability and is generally considered to be non-corrosive to carbon steel. In pure form it is colorless, but commercially available sulfolane may exhibit a dark or amber color.
Sulfolane is applied in a number of liquid-liquid and liquid-vapor extractions, and is particularly suitable for the extraction of aromatic compounds such as benzene, toluene and o,m & p-xylenes from hydrocarbon streams having a carbon number from about C6 to about C10. These aromatic compounds typically are not plentiful in crude petroleum, but are produced by thermal or catalytic reforming. Many catalytic reforming processes use a platinum-containing heterogeneous catalyst, and up to 1% of a halogen may be used as a promoter to regulate the acidity at the cracking and isomerization sites of the support, which typically is alumina granules. During catalytic reforming a number of chemical conversions occur, e.g., paraffins may be cracked and hydrogenated, or isomerized, or undergo dehydrocyclization; naphthenes with side chains may undergo dehydrocyclization, and naphthene rings are dehydrogenated into aromatic rings. Thus by selecting feed fractions that are rich in naphthenes it is possible to produce a reformate that contains 35-60% of benzenes, toluene and the xylenes. These aromatic compounds may also be extracted from, e.g., hydrotreated pyrolysis naphtha feeds.
In the course of the extraction process, impurities find their way into the sulfolane solvent, possibly from other processing steps upstream of the extraction process, and possibly from decomposition of the sulfolane. The decomposition may have been thermal or catalyzed by the various chemicals present or added to the upstream processes. These impurities can cumulate and concentrate in the sulfolane and lead to significant corrosion rates of the processing equipment used in the extraction process, and if not remedied, result in serious operating and safety problems. The use of pH control, i.e., maintaining the pH value of the sulfolane above about 8.5 by the addition of basic materials and/or corrosion inhibitors such as amines, particularly alkanolamines such as monoethanol amine, has met with but limited success in alleviating this serious problem.
A procedure has now been found which substantially reduces the level of corrosive impurities in the sulfolane at low cost and without requiring substantial amounts of costly process equipment.